EP0812075B1 - Optical fibre transmission systems including dispersion measurement and compensation - Google Patents

Optical fibre transmission systems including dispersion measurement and compensation Download PDF

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Publication number
EP0812075B1
EP0812075B1 EP19970303261 EP97303261A EP0812075B1 EP 0812075 B1 EP0812075 B1 EP 0812075B1 EP 19970303261 EP19970303261 EP 19970303261 EP 97303261 A EP97303261 A EP 97303261A EP 0812075 B1 EP0812075 B1 EP 0812075B1
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Prior art keywords
dispersion
optical
signal
transmission system
optical path
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German (de)
English (en)
French (fr)
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EP0812075A3 (en
EP0812075A2 (en
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Kim Bryron Roberts
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Nortel Networks Ltd
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Nortel Networks Ltd
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Priority claimed from US08/660,565 external-priority patent/US6252692B1/en
Priority claimed from GBGB9615284.8A external-priority patent/GB9615284D0/en
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Publication of EP0812075A2 publication Critical patent/EP0812075A2/en
Publication of EP0812075A3 publication Critical patent/EP0812075A3/en
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/25—Arrangements specific to fibre transmission
    • H04B10/2507—Arrangements specific to fibre transmission for the reduction or elimination of distortion or dispersion
    • H04B10/2513—Arrangements specific to fibre transmission for the reduction or elimination of distortion or dispersion due to chromatic dispersion

Definitions

  • This invention relates to optical transmission systems, to control systems for optical transmission systems, to dispersion measurement systems, and to elements for receiving or processing signals in optical transmission systems, and to methods of transmitting data along an optical path.
  • optical terminals of optical fibre transmission systems is limited by the optical power that can be launched into optical fibre by optical transmitters of the optical terminals, the loss and dispersion of optical fibre interconnecting the optical terminals, and the sensitivity of optical receivers of the optical terminals.
  • optoelectronic repeaters Where the distance between desired end points of an optical fibre transmission system exceeds the maximum distance between optical terminals, optoelectronic repeaters have been provided.
  • Each optoelectronic repeater comprises an optical receiver for converting the optical signal, to an electrical signal, electronics for regenerating the electrical signal, and an optical transmitter for converting the regenerated electrical signal to an optical signal for transmission to the next optoelectronic repeater or to a terminal of the system.
  • the optical signals are optically demultiplexed at each repeater, so that the signal at each distinct wavelength is coupled to a respective optical receiver for conversion to a respective electrical signal, each respective signal is applied to a respective optical transmitter operating at a distinct wavelength, and the transmitted signals are optically multiplexed for transmission to the next optoelectronic repeater or to a terminal of the system
  • Optical amplifiers for example Erbium Doped Fibre Amplifiers (EDFAs), amplify optical signals directly without converting them to electrical signals. Because EDFAs do not require high speed regeneration electronics, they can be cheaper than optoelectronic repeaters for high speed optical fibre transmission systems.
  • EDFAs Erbium Doped Fibre Amplifiers
  • the EDFAs can amplify optical signals at multiple wavelengths without optically demultiplexing them, thereby avoiding the costs of optical multiplexing and demultiplexing, and the costs of multiple optical receivers, multiple regeneration circuits and multiple optical transmitters. Consequently, EDFAs can also be cheaper than optoelectronic repeaters for WDM systems.
  • a regenerator may be necessary after several optical amplifier stages, to rebuild the data signal and remove the noise and dispersion degradation.
  • EP-A-0140853 discloses a method of measuring intermodal dispersion in a multimode optical fibre.
  • dispersion also known as Group Velocity Dispersion, in fibre at least, occurs as a result of two mechanisms:
  • Dispersion is the derivative of the time delay of the optical path with respect to wavelength.
  • the effect of dispersion is measured in picoseconds arrival time spread per nanometre line width' per kilometre length (ps nm -1 km -1 ).
  • the magnitude of intramodal and material dispersions both vary with wavelength, and at some frequencies the two effects act in opposite senses. It is generally possible, on a given single mode fibre, to find a wavelength around which there is negligible dispersion, or, conversely, to design a fibre to have minimum dispersion at a desired wavelength. References to dispersion herein will mean the sum total of group velocity dispersion effects.
  • Dispersion in optical fibre presents serious problems when using light sources whose spectrum is non-ideal for example broad or multispectral-line, or when high data rates are required, e.g. over 2 GB/s.
  • This problem has previously been addressed, at least partially, in four ways. Firstly, by operating at or close to the optical frequency at which the dispersion is a minimum, for example at a wavelength of 1.3 micron in conventional silica fibre. The frequency does not generally correspond with the frequency of minimum transmission loss and attempts to modify the fibre to shift its frequency of minimum dispersion usually result in some loss penalty.
  • This solution has limitations for two reason. Firstly manufacturing variations will always occur. Secondly, a non linearity called four wave mixing seriously degrades WDM signals near the dispersion zero of one piece of fibre. Accordingly, it may be preferable to operate in a given region of dispersion which may not include the dispersion zero.
  • the second way of overcoming the problem is to use a source with a near ideal narrow linewidth spectrum.
  • the limits for improvement in this respect have been reached since at higher bit rates, the Kerr effect becomes significant. This is where the index of refraction varies with intensity, which causes self phase modulation, or cross phase modulation.
  • the resulting frequency redistribution means that dispersive degradation increases again.
  • dispersion compensators have been used to equalise the dispersion with an element of equal and opposite dispersion.
  • Such dispersion compensators may take the form of length of fibre, a Mach Zehnder interferometer, an optical resonator, or a Bragg reflector. Some of these compensators can give a variable, controllable amount of compensation.
  • a fourth technique is to change the modulation at the transmitting end.
  • Dispersion produces an FM to AM conversion effect which can facilitate bit detection and thereby extend transmission distance without controlling or compensating dispersion.
  • the dispersion causes shifting of adjacent signal components of different wavelengths, resulting in either energy voids or energy overlaps at the bit transitions. Constructive interference in an overlap causes a positive peak in the optical signal, while a void produces a negative peak. These positive and negative peaks represent an AM signal which may be detected to reproduce the original bit stream.
  • the document proposes the additional step of adjusting the output power of one or more of the inline amplifiers to further stabilise the dispersion-induced optical signal energy voids and overlaps and thereby further improve the detection thereof.
  • This method requires difficult precision engineering and so is impractical for commercial exploitation.
  • dispersion-shifted fibre With the different types of dispersion-shifted fibre, dispersion compensating fibre, and dispersion-compensating filters that could make up a given link, determining the dispersion of a link is no longer the simple operation of multiplying the length in km by the 17 ps/nm/km dispersion characteristic of standard single mode fibre. Moreover, when there are optical switches or controllable optical dispersion compensators in the link the dispersion can change as a function of time.
  • Reference numeral 41 is an optical transmitter
  • 42 is an optical receiver
  • 43 is an optical fibre
  • 44 is a tuneable light source
  • 45 is a tuneable filter
  • 46 and 47 are optical amplifiers
  • 48 is an optical detector
  • 49 is a drive circuit
  • 50 is a tuneable filter
  • 51 is a repeater
  • 52 is a sweep controller
  • 53 is a transmission characteristic measuring section.
  • the drive circuit 49 is controlled by the sweep controller 52 to sweep the emission wavelength of the tuneable light source 44.
  • sweeping can be accomplished by varying the currents Ip and Id; in the case of a semiconductor laser of other configuration, sweeping of the emission wavelength can be accomplished by continuously varying the temperature.
  • the optical signal with the thus swept emission wavelength is transmitted along the optical fibre 43 and via the repeaters 51, and is detected by the optical detector 48 of the optical receiver 42, where the received result is applied to the transmission characteristic measuring section 53 which measures the transmission characteristic between the optical transmitter 11 and the optical receiver 12. Based on the result of the transmission characteristic measurement, the emission wavelength of the tuneable light source 44 and the wavelength transmission characteristics of the tuneable filters 45 and 50 are so set as to achieve the best transmission characteristic.
  • Variable dispersion compensators may also be controlled to find an optimum measured transmission characteristic.
  • the transmission characteristic measuring section 53 may be constructed to measure the transmission characteristic by measuring bit error rates (BER). Alternatively, it may be contructed to measure the transmission characteristic using an eye pattern.
  • BER bit error rates
  • the emission wavelength of the tuneable light source 44 may be adjusted so that the eye pattern opens widest.
  • control may be performed manually while observing the eye pattern, or alternatively, automatic control by means of computer processing may be employed.
  • An alternative way of measuring the bit-error rate is to measure the Q value (electrical SNR).
  • the bit-error rate given by the Q value agrees with the minimum-value of the actually measured bit-error rate.
  • Other methods such as measuring the transmitted waveform and using specifications of equal bit-error rate curves, may also be employed.
  • EP-A-0684709 discloses an optical communication system which also uses a transmission characteristic such as the eye margin or the bit error rate to optimise the dispersion.
  • the dispersion optimising system comprises adjustable dispersion compensating fibres to compensate for dispersion in system fibres.
  • the amount of dispersion introduced by the dispersion compensating fibres is varied depending upon the amount of compensation required as determined from the eye margin or the bit error rate.
  • US 4 677 618 by IBM addresses the problem of relative delay between different wavelength sources for the specialised case of transmitting data words in parallel using optical sources of different wavelengths for different bits of the word.
  • the relative delay is the integral of the dispersion between respective wavelengths.
  • the relative delay is measured at the receiving end then used to restore the original timing alignment of the bits of each word by adding appropriate compensating delays. There is no measurement of the dispersion, i.e. the derivative of the time delay of the optical path with respect to wavelength.
  • the present invention seeks to improve on such techniques.
  • an optical transmission system for transmitting data along an optical path
  • the transmission system comprising: a controllable element; characterised in that the optical transmission system further comprises means for determining a group velocity dispersion of at least part of the optical path, said dispersion determining means comprising means for determining timing jitter in a signal after passage of said signal along said at least part of the optical path, and means for deriving a group velocity dispersion value from the timing jitter; and means for controlling the element in the transmission system in dependence on the determined group velocity dispersion value.
  • Measuring the dispersion directly enables better control of dispersion compensators, or other system elements, or improved monitoring and fault isolation.
  • Measuring the dispersion rather than resulting degradation on the eye and BER enables a margin of performance of the optical path to be assessed under realistic zero error conditions.
  • a control system for a controllable element of an optical transmission system comprising means for controlling the controllable element of the optical transmission system, characterised in that the control system further comprises means for determining the group velocity dispersion of at least a part of an optical path of the transmission system, said dispersion determining means comprising means for determining timing jitter in a transmitted signal after passage of said signal along said at least part of the optical path, and means for deriving a group velocity dispersion value from the timing jitter, and wherein the control means is operable in dependence on the determined group velocity dispersion value.
  • a dispersion measurement system characterised in that the system is for measuring a group velocity dispersion of at least part of an optical path of an optical transmission system, the measurement system comprising at least a portion of an element of the transmission system for receiving or processing a signal passed along the at least part of the optical path, said portion of said element comprising dispersion determining means comprising means for determining timing jitter in the signal after passage of said signal along said at least part of the optical path.
  • lntergrating portions of the dispersion measurement means with portions of existing elements in the transmission system enables a reduction in the total amount of hardware required. Furthermore, it enables measurements under realistic operating conditions, at the operating wavelengths, while data traffic is present, which is particularly important where variable dispersion elements are present.
  • a preferred feature involves the dispersion value being used to control dispersion compensating means or transmitters in the transmission system. This enables better control of such system elements, and easier design of control algorithms using linear control, than the case where values used to control elements are dependent on numerous system parameters.
  • Another preferred feature provides a monitoring means for comparing the optical dispersion value to a threshold. This enables dispersion problems to be flagged and isolated easily, to facilitate adjustment of variable compensators, or repair or replacement, by less skilled staff, either when commissioning or operating the transmission system. This has not been possible up to now. Where the threshold is exceeded measures may be taken to alter the data traffic, to avoid or reduce bit errors.
  • Centralised fault monitoring, or control of the transmission system can be facilitated by sending the measured value to a remote monitoring location. This can reduce costs. Measuring the timing jitter in the signal is advantageous as it can be accurately measured, and is linearly related to dispersion, as it results from variation in transmitted wavelength.
  • the timing jitter is derived from a clock recovered from the received signal. This takes advantage of equipment already provided in the receiver, thus saving costs.
  • the jitter is derived from the phase difference between the recovered clock and the output of a phase locked loop locked to the recovered clock. This takes advantage of circuitry already provided in the receiver and the regenerator, thus saves costs, which is particularly significant for high speed circuitry.
  • phase locked loop which is already used to provide a clock for a regenerator is preferred over the loop in the clock recovery circuit, because the regenerator loop may already be provided with filtering which passes only low frequencies. This means it will not respond to jitter above such frequencies. Thus such jitter will appear as a phase difference, and thus can be separated from the clock signal.
  • the measuring system also comprises a means for applying a variation in wavelength in the form of a predetermined pattern, to the signal before it passes along the part of the optical path to be measured. This enables easier extraction of the resulting timing jitter from the received signal, and thus more accurate measurement.
  • the wavelength variation is also used for monitoring or altering other parameters of the transmission system.
  • Dual use saves costs, enables the dispersion measurement function to be retrofitted to existing systems, and minimises interference such as beating, which may be caused by added wavelength variations.
  • the wavelength variation is a low frequency dither signal for suppressing stimulated Brillouin scattering.
  • Such variation may already be used in a transmission system particularly a high speed system, and can conveniently be reused for measuring dispersion.
  • a correlation is carried out between the measured timing jitter and the predetermined pattern. This enables more accurate measurement.
  • the pattern is asymmetric and the sign of the dispersion can be determined. This enables more information about the dispersion to be obtained, which is useful in particular in determining which way to adjust a variable compensator, to reduce the dispersion.
  • the dither pattern is pseudo random. This can minimise any interference the pattern may cause to the data traffic, or to other elements in the system.
  • Another aspect of the invention provides a method of transmitting data along an optical path in an optical transmission system, characterised in that the method comprises: the step of determining a group velocity dispersion in at least part of the optical path at the same time as the data is being transmitted, said step of determining a group velocity dispersion comprising determining timing jitter in a signal comprising the transmitted data after passage of said signal along said at least part of the optical path, and deriving a group velocity dispersion value from the timing jitter; and using the determined group velocity dispersion value for monitoring whether it exceeds a threshold or for controlling data flow, or for controlling a dispersion compensating device or method.
  • Figure 2 shows elements of an optical transmission system, comprising a transmitter (1) optical amplifier (2), receiver (3), regenerator (4) and elements of a dispersion measurement system (5) incorporated in the transmitter the regenerator and the receiver.
  • Electrical signals are multiplexed and converted into optical form for transmission along optical fibre.
  • One or more optical amplifier stages may be necessary before the receiver is reached.
  • the regenerator may process electrical signals received from the receiver for further transmission optically, or for use in electrical form.
  • regenerator covers variants with electrical demultiplexing functions, such as a terminal or an add/drop multiplexer (mux).
  • the dispersion measurement means may be separate from, or incorporated into the receiver or the amplifier or any other element in the transmission path.
  • the measurement system can take advantage of patterns in the normal data traffic. Secondly it may use predetermined patterns impressed on the data traffic (with reference to Figures 3 and 4). Thirdly it may use patterns impressed on the optical carrier (with reference to Figures 5 and 6).
  • Figure 3 shows one possible arrangement of the transmitter 1 for use in the system of Figure 2.
  • a predetermined pattern is generated and fed to a data multiplexer for transmission together with other data traffic.
  • the pattern generator output is used to modify one or more of the data channels prior to multiplexing.
  • the modulators can be optical or be implemented in two stages; a digital electronic stage to impress the pattern on the data traffic, followed by an optical modulation stage. This enables processing to be carried out at lower data rates, and thus components are likely to be cheaper.
  • a further alternative transmitter 1 is shown for use in the system of Figure 2.
  • the pattern generator is used to modulate an optical carrier, which is subsequently externally modulated by the data traffic.
  • Direct modulation by the data traffic of the optical source usually a laser
  • Advantages of using direct modulation for the pattern include the reduced cost of the parts required, particularly where low frequencies are involved, and the amount of wavelength variation which can be achieved.
  • FIG. 6 is a block schematic diagram of the optical transmitter of Figure 5 for adding the predetermined pattern, in the form of a dither.
  • the optical transmitter comprises an optical source in the form of a semiconductor laser 1510, a signal modulation arrangement 1520 in the form of a line encoder 1522 and an external optical modulator 1524, and a dither modulation arrangement 1530.
  • the dither modulation arrangement 1530 comprises an optical tap 1532, an opto-electronic conversion device in the form of a PIN diode 1534, a calculation arrangement 1540, and a dither amplitude control arrangement 1550.
  • the calculation arrangement 1540 comprises a transimpendance amplifier 1542, a first analog to digital converter 1543, an AC amplifier 1544, a sample and hold circuit 1545, a second analog to digital converter 1546, and a microcontroller 1548.
  • the dither amplitude control arrangement 1550 comprises a digital to analog converter 1552, a chopper 1554, a bandpass filter 1556 and a voltage controlled current source 1558.
  • the digital to analog converter 1552 applies an analog signal to a signal input of the chopper 1554, the analog signal having a signal level corresponding to a digital code supplied to the digital to analog converter 1552 by the microcontroller 1548.
  • the microcontroller 1548 repeatedly applies a 64 bit Miller encoded pseudorandom sequence to a control input of the chopper 1554 to modulate the analog signal at 64 kbps.
  • the modulated signal is filtered by the bandpass filter 1556 and applied to a control input of the voltage controlled current source 1558 to modulate a bias current of the semiconductor laser 1510. Consequently, the semiconductor laser 1510 emits an optical signal modulated by a 64 kbps low modulation index dither signal. This gives a dither with predominant frequency content between 20 and 40 kHz.
  • the dither modulated optical signal is applied to the external optical modulator 1524 which responds to a high speed (approximately 2.5 Gbps) electrical data signal supplied by the line encoder 1522 to superimpose a high speed, high modulation index data modulation on the dither modulation of the optical signal.
  • the twice modulated optical signal is coupled to an output fibre 1526 of the optical transmitter 1500.
  • the optical tap 1532 couples approximately 3% of the modulated optical signal on the output fibre 1526 to the PIN diode 1534.
  • the PIN diode 1534 converts the tapped optical signal to a photocurrent, and the transimpedance amplifier 1542 amplifies and converts the photocurrent to a voltage.
  • the first analog to digital converter 1543 converts the analog voltage to digital code which estimates the total optical power of the tapped optical signal, and this digital code is supplied to the microcontroller 1548.
  • the AC amplifier 1544 is AC coupled to the transimpedance amplifier 1542 and provides further amplification of an AC component of the voltage.
  • the sample and hold circuit 1545 samples the amplified AC voltage.
  • the second analog to digital converter 1546 converts the analog samples to digital codes which are supplied to the microcontroller 1548.
  • the microcontroller 1548 correlates the digitally encoded AC signal with the pseudorandom sequence applied to the chopper 1554 to compute the amplitude of the dither modulation in the tapped optical signal, compares the dither modulation amplitude to the estimated total power of the tapped optical signal to compute the dither modulation depth, and adjusts the digital code applied to the digital to analog converter to fix the dither modulation depth at a known and precisely controlled value.
  • the A.M to F.M. conversion characteristic of each particular laser is calibrated within the memory associated with the microcontroller, to improve the precision of the resulting F.M.. The characteristic can be measured easily for any laser.
  • All the patterns and circuitry for generating the dither may be present for monitoring or altering other parameters of the system such as suppression of stimulated Brillouin scattering.
  • An example of this is disclosed in US-A-5513029 by Roberts and Habel.
  • the present invention can make use of such transmitters with little or no modification.
  • the same circuitry can be used for noise monitoring for example.
  • the pattern may be Miller encoded to help shape the spectral shape. It may be pseudorandom so as to minimise any correlation with other useful information in the transmitted signal, to avoid interference such as beating.
  • the pattern may be asymmetric so as to enable the sign of dispersion to be determined, as will be explained below.
  • ⁇ t D ⁇ ⁇ ⁇ + k ⁇ ⁇ ⁇ + noise
  • the amount of amplitude modulation at the transmitter side can be selected, to achieve a compromise between the requirements of accuracy of measurement, and minimizing undesired side effects such as jitter and eye closure.
  • a larger amplitude can give better accuracy, but with worse side effects.
  • 0.6% R.M.S. amplitude variation produced 700 MHz of frequency variation, though the precise relationship is laser dependent.
  • the resulting jitter for a 100 km length of single mode fibre is in the order of 10 picoseconds. This represents one fortieth of the clock period, and so cannot easily be measured directly, but is obtainable using correlation techniques.
  • Figure 7 shows a schematic diagram of a receiver (3) for use in the system of Figure 2. It may serve as the end point of the link, or as a part of a repeater. It includes an optical to electrical converter (70) an amplification and filtering stage (71), a clock recovery stage (72), a threshold stage (73) and a retiming stage (74).
  • the clock recovery stage is shown in more detail in Figure 8. It is well known how to put each of these stages into practice, and so it is not necessary to describe particular circuits.
  • the outputs of the receiver include digital data signals and clock signals.
  • the clock signals may be divided down so as to reduce the clock rate.
  • the data signals may be demultiplexed or converted from serial to parallel format. Measures such as these, for slowing the clock rate, make the data signals easier and cheaper to process.
  • Means for measuring the dispersion in the optical path can be incorporated in the receiver, though this has not been illustrated.
  • An optical tap at the input to the receiver could feed a small proportion, for example 5% of the signal to measurement circuitry (not shown).
  • Such measurement circuitry could also use signals taken from any of the receiver stages shown in Figure 7.
  • Figure 8 shows a typical clock recovery circuit using a phase locked loop.
  • An alternative possibility is to make use of a SAW filter (not shown). Again, in principle, the measurement of dispersion can us signals from various stages in the clock recovery circuit.
  • the bandwidth of the phase locked loop shown in Figure 8 is typically 200 to 5000 times less than the bit rate of the data traffic.
  • the clock recovery phase locked loop bandwidth might be 1 MHz. This is designed to enable it to track substantially all incoming jitter, so as not to degrade the B.E.R. This means it will pass the jitter caused by the predetermined pattern, without significant attenuation. This is in contrast with the phase locked loop of the regenerator, as will be described later.
  • Figures 9 and 10 show examples of regenerators, where the dispersion is derived from the recovered clock.
  • the timing jitter is separated using a phase locked loop in the regenerator.
  • a correlation means then converts this jitter into a dispersion value.
  • the two alternative regenerator circuits shown in Figures 9 and 10 differ only in that the dispersion conversion means shown in Figure 9 uses an original pattern generator, while that of Figure 10 derives the pattern from the recovered data.
  • a separate clock to that used by the receiver is generated, and locked to the incoming clock from the receiver, using a phase locked loop.
  • This loop comprises a phase difference detector 91, a narrow band filter 92, a voltage controlled oscillator (VCO) 93 and a clock divider 94.
  • VCO voltage controlled oscillator
  • timing jitter on the recovered clock fed into the regenerator notably the bulk of the jitter resulting from the pattern, will not pass through the phase locked loop, which typically has a 10 kHz low pass bandwidth. This is designed to be high enough for good acquisition behaviour, without passing on jitter. Therefore this portion of the timing jitter will appear as a phase difference between the clock output by the VCO, and the recovered clock input from the receiver. This difference is detected by the phase difference detector 91.
  • this timing jitter should be correlated with the original pattern to remove parts of the timing jitter caused by other parts of the transmitted signal, and noise. This will leave the timing jitter caused by the pattern, timing jitter caused by the phase response of circuitry between the optical path and the measurement means, (principally amplification stages) and some noise.
  • the allowable range of dispersion becomes smaller, and thus the measurement of the dispersion must become more accurate.
  • dispersion compensators may require greater accuracy, particularly if they have complex responses, or narrow ranges of linear response.
  • timing measurements using some form of correlation with a predetermined pattern of wavelength variation can give good results.
  • the timing measurements are so small that noise is a significant factor, which needs to be filtered out.
  • noise is a significant factor, which needs to be filtered out.
  • the receiver and anywhere between the optical path and the measurement means, distortions and noise may be introduced.
  • the wavelength variations, and consequential timing jitter should be small enough to avoid interference with data traffic.
  • FIG. 11 A more detailed schematic diagram showing an example of the correlation to separate the desired part of the timing jitter is shown in Figure 11.
  • FIG 11 shows in functional terms various stages in converting the timing jitter in the form of a clock phase difference signal, into dispersion value.
  • DSP digital signalling processor
  • the clock phase difference signal could be transmitted elsewhere, and the conversion process to obtain a dispersion value, could be carried out in some means provided elsewhere. This possibility is encompassed in the present invention.
  • the disadvantage with such an arrangement is the additional hardware which may be required in transmitting the clock phase difference before converting it into a dispersion value.
  • the additional hardware may be required because the clock phase difference signal would typically require a 200 K Byte per second transmission channel, well beyond the bandwidth of a currently available modem.
  • the first stage is a bandpass filter, so as to restrict the signal to be processed, to only the frequencies of interest, according to the type of wavelength variation induced at the transmitter.
  • a fast Fourrier Transform FFT
  • correlation can be carried out in the time domain, but frequency domain is usual.
  • a multiplication stage is then provided, to multiply the FFT output with a stored frequency template representing the pattern to which the timing jitter is being correlated. How this template is derived will be described later.
  • the result of the multiplication is put through an inverse FFT process to return to the time domain.
  • a series of complex samples is output, ready for interpolation. Interpolation improves accuracy, but may not be necessary depending on the number of samples and the accuracy required.
  • the interpolation stage involves interpolating between complex samples to determine a peak correlation.
  • This can be implemented in various ways. Many algorithrns have been developed, particularly for radar use. Which of these well known methods will give best results depends on the particular system characteristics, in particular on the amount of noise, the number of samples relative to the pattern, the shape of the auto correlation of the pattern, the desired accuracy available computing power, and desired speed of obtaining a result. Also, the determination of sign of the dispersion affects the interpolation as described below.
  • phase invariant detection To find the sign of the dispersion, an asymmetric pattern must be used if phase invariant detection is used, as described above.
  • the peak detection needs to find the largest absolute value and report the corresponding signed value, which gives the sign of the dispersion.
  • noise reduction filter may be employed before or after the receiver circuitry compensation, to reduce noise not removed by the correlation process.
  • Generating the frequency template for the above correlation process may be carried out based on an original predetermined pattern as shown in Figure 11, or may be based on data patterns extracted from the transmitted data, as indicated in Figure 10.
  • the former technique has the advantages of enabling preprocessing and not using up transmission bandwidth. Also, it does not require demultiplexing of data which may involve additional hardware.
  • the pattern is converted into the frequency domain by an FFT, then the complex conjugate is derived, to prepare for the multiplication. Compression for storage may then be necessary. It is then ready as a template for multiplying with successive frequency domain representations of the clock phase difference.
  • the amplitude of the part of the timing jitter which correlates to the impressed pattern will be directly proportional to the dispersion, subject to the effects of receiver electronics, and noise.
  • timing jitter which is measured and converted, could be derived from the data rather than the recovered clock.
  • An original impressed data pattern could be correlated with this measured data timing jitter. It would also be conceivable to measure and use the timing jitter caused not by any impressed data pattern, but by the normal data traffic.
  • the pattern for the correlation template would need to be extracted from this normal data traffic as shown in Figure 10.
  • the timing jitter could be measured either from the recovered clock, as shown in Figure 10, or from the data (not shown).

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EP19970303261 1996-06-07 1997-05-13 Optical fibre transmission systems including dispersion measurement and compensation Expired - Lifetime EP0812075B1 (en)

Applications Claiming Priority (4)

Application Number Priority Date Filing Date Title
US660565 1996-06-07
US08/660,565 US6252692B1 (en) 1996-06-07 1996-06-07 Optical fibre transmission systems
GB9615284 1996-07-20
GBGB9615284.8A GB9615284D0 (en) 1996-07-20 1996-07-20 Optical fibre transmission systems

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EP0812075A2 EP0812075A2 (en) 1997-12-10
EP0812075A3 EP0812075A3 (en) 2000-03-15
EP0812075B1 true EP0812075B1 (en) 2006-04-12

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DE69735660T2 (de) 2006-08-24
JPH1084317A (ja) 1998-03-31

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